A device of this kind is used, for example, to determine the shape of the bezel of a frame rim, i.e. the groove that runs around the interior of the frame rim and retains an ophthalmic lens in the frame rim. To this end, the feeler is inserted into the bezel and follows the contour of the bezel while the device measures the coordinates of the position of the feeler along its path, thereby storing a digital image of the contour of the bezel. An ophthalmic lens blank can then be trimmed to the dimensions of the bezel so that it can be inserted perfectly into the frame rim.
A contour reading device of the above kind is known from the document U.S. Pat. No. 5,121,550. The device includes a support for holding an article to be palpated. A circular platform is mounted to turn relative to the support and carries a reading subassembly that includes a feeler. The feeler includes a rod and a finger extending from said rod. The distal end of the finger is adapted to move along the contour.
The reading subassembly includes a slideway on which a mobile carriage attached to the feeler is mounted.
This device measures polar coordinates along the path of the feeler where the angular dimension (θ) corresponds to the rotation of the rotary platform and the radial dimension (ρ) corresponds to the movement in translation of the carriage on the slideway.
The finger moves in the bottom of the bezel of the rim and remains in contact with the latter because of the effect of a radial force applied to hold the finger against the bezel.
The article to be palpated being in three dimensions, the feeler must be able to move along an axis (z) corresponding to a movement transverse to the plane comprising the dimensions (ρ, θ).
These devices include a motor initially driving the feeler along the axis (z) until it faces the bezel. Once the finger has been brought into contact with the bezel, by movement in translation along the slideway, said motor is disengaged or passive so that, during the reading of the contour, movement along the axis (z) is unimpeded.
If the article to be read is a frame rim having moderate curvature, the movement of the feeler along the axis (z) is of small amplitude and the finger easily remains in contact with the bottom of the bezel.
On the other hand, in the case of frame rims with highly curved portions, where the amplitude along the axis (z) is therefore high, the free movement of the finger along the axis (z) makes it more difficult for the finger to remain in the bottom of the bezel.
To limit the risk of movement away from the bottom of the bezel, it is necessary to reduce the rotation speed when the finger includes a highly curved portion, for example.
There is also known from the document U.S. Pat. No. 6,325,700 a contour reading device including calculation means for predicting the amplitude of the contour to be read in three directions, and in particular along the axis (z), with the aim of preventing the finger moving away from the bottom of the bezel. These calculation means estimate the evolution along the axis (z) of the portion to be traveled as a function of the measured coordinates of the portion already traveled. A motor for positioning the feeler along the axis (z) is controlled as a function of the calculated estimate. The feeler is guided as it follows the contour but there is nothing to guarantee that the finger will remain in the bottom of the bezel.